In making a hole or a bore in a workpiece, firstly, boring machining is roughly performed for the workpiece using a drill, and then, cutting machining is performed using a reamer.
Constitutive materials adopted for the drill or the reamer include, for example, SK material, SKD material, or SKH material defined by Japan Industrial Standard (so-called high speed tool steel) containing high carbon steel as a major component, super alloy material such as nickel-based alloy and cobalt-based alloy, and superhard material as composite material of ceramics and metal. In order to improve abrasion resistance, the surface of the drill or the reamer is sometimes coated with a coating film of hard ceramics such as TiC or TiN.
The high speed tool steel and the super alloy material have high strength and high toughness. However, the high speed tool steel and the super alloy do not have sufficient abrasion resistance, compressive strength, and rigidity. The superhard material has high abrasion resistance, compressive strength, and rigidity. However, the superhard material does not have sufficient toughness and tends to cause cracks and breakage. That is, the characteristics of the high speed tool steel and the super alloy material are opposite to the characteristics of the superhard material. Therefore, the constitutive material for the drill or the reamer is selected in consideration of the constitutive material of a workpiece and the magnitude of a variety of stresses exerted on the drill or the reamer when the boring machining or the cutting machining is performed. Such stresses include compressive stress exerted when the pressing force is applied to the workpiece, tensile stress acting on the leading part and the cutting part, and tensile stress exerted between the portion to be machined and the portion not to be machined.
It is desirable that the drill or the reamer has high hardness, high strength, and high toughness. High hardness, i.e., high abrasion resistance is essential for a long service life. High strength helps to prevent deformation of the drill or the reamer, even if the stresses as described above are exerted thereon. Further, the drill or the reamer having high toughness scarcely suffers from the occurrence of cracks and breakage. However, conventional drills or reamers do not have all of the characteristics described above.
For example, in the case of the drill or the reamer composed of a superhard material, it is possible to improve toughness by increasing the composition ratio of metal. However, the superhard material having high metal composition ratio does not have high hardness and strength. Therefore, the service life of the drill or the reamer may not be long. In contrast, it is possible to improve hardness and strength by decreasing the composition ratio of metal at the sacrifice of toughness. However, the cracks and breakage tend to occur more frequently.
As described above, the superhard material having high hardness and strength does not have high toughness. The superhard material having high toughness does not have high hardness and strength. Therefore, it is difficult to improve all of the characteristics (hardness, strength, and toughness) of the drill or the reamer.